Spatial optical memory

ABSTRACT

A device for storing photons between one and a plurality of reflected surfaces, and the storage of information, in particular in the form of digital data, being provided by creating at least one circulating memory using the storage device as a delay line. Each device consists of one or a plurality of photon sources, using any wavelength of light, one or a plurality of photon detectors and one or a plurality of reflective surfaces. The injected photons are delayed from reaching the output by one or a plurality of reflections from one surface of the device to the same or another surface of the device, each reflection extending the distance traveled for each photon, thereby inducing a delay.

FIELD OF THE INVENTION

The present invention relates in general to optical data storage devicesand methods, and concerns the storage of photons by means of reflectionbetween one or a plurality of highly reflected surfaces.

BACKGROUND OF THE INVENTION AND RELATED ART

Typically the storage of photons has an analogy to electron storage suchas the well known capacitor memory cell. Difficulty in producing suchdevices has arisen from the physical problems of containing a photon ora stream of photons. To date most approaches for storing photons areeither based on the effects of optically bistability or the utilizationof long fiber loops.

Devices that utilize the use of optical bistability are bothtechnologically complex and expensive, with much of the early workmotivated by the notion that optics could be used to avoid some of theintrinsic speed limitations of electronic storage analogs. These devicesrely on electro-optic conversion and hence the storage of an electron.Devices that utilize the use of fiber loops exist in the form ofsequential storage pipes used to house a sequential stream of photonsflowing in a re-circulating path in a first in first out fashion. Thesefiber loops are used as delay lines and are typically referred to as“Programmable Photonic Fiber Loop Memory”, A. Dickson et al. Proceedingsof the 16th Australian Conference on Optical Future Technology, pp274-277,1991.

For future data storage, it is desirable to have a photon storage deviceas a counter part or replacement for today's analogous electronic memorydevices. These should be capable of meeting the high speed data storagerequirements expected for the next generation of information processinghardware. In addition to speed, such devices should also meet theexpected high storage capacity (bit density) requirements for advancedprocessing applications. Finally the importance of error reduction fromlimited alpha emissions as well as radiation hardened capability wouldbe desirable for space exploration.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus foroptically storing photons for the purpose of carrying information.

This objective has been accomplished by one or a plurality of photonsources injecting a plurality of photons into the entrance of the saidstorage device. Once inside the device, the injected photons are, wherenecessary, focused into a region that allows for the said photons to bereflected a plurality of times from one highly reflective surface toanother highly reflective surface. The number of complete reflectionswithin a device will vary depending on device geometry, with eachreflection extending the total distance traveled for each injectedphoton within the said device. On exiting the said device, eachreflected photon is, where necessary de-focused, and selectivelydetected by one or a plurality of photon detectors.

The principles of the present invention allow for the construction ofdevices and systems which should be capable of delivering high speed,high bit density and radiation hard storage requirements expected forthe next generations of information processing hardware. Moreover, indoing so, these principles do not require a light source of a specificfrequency, or photon sources and detectors of a specific kind, except tothe extent that compatibility between the photon source and detectormust be maintained, as known in the art.

BRIEF DESCRIPTION OF DRAWINGS

For a more complete understanding of the present invention, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings in which:

FIG. 1. shows a block diagram for the said storage device.

FIG. 2. shows the said storage device input, and focus control unit.

FIG. 2 a. shows the relationship between the angle of incidence and theangle of reflection on surface.

FIG. 2 b. shows a variation of design whereby two curved surfaces areused to comprise the focus control unit.

FIG. 2 c shows a variation of design for the focus control unit using apartial mirror.

FIG. 3. shows details of the first photon reflection off a curvedsurface.

FIG. 4. shows details of the second photon reflection of a flat surface.

FIG. 5. shows photons entering the Plateau region. For clarity, the fullstream of photons is not shown in the figure. Instead, photons at thebeginning and the end of the Plateau region are shown.

FIG. 5 a. shows photon propagation within the plateau region.

FIG. 6. shows the output of the said device, the photon detector and theoutput focus control unit.

FIG. 7. shows the three different types of reflective surfaces than canbe employed by the said device.

FIGS. 7 a, b and c show different surface configurations for the saiddevice.

GENERAL DESCRIPTION

The principles of the present invention and their advantages are bestunderstood by referring to the illustrated embodiment depicted in FIGS.1-7 of the drawings, in which like numbers designate like parts.

For the sake of simplicity, it is assumed that all reflected surfacescontained herein, unless otherwise stated, are deemed to be that of ahigh reflectance, essentially reflecting 100% of all incident photons.

For the sake of simplicity, it is further assumed in the following, thata light pulse represents one bit of information. This is not necessarilyso, but for most applications this approach is reasonable.

For the sake of simplicity, it is further assumed that a surfacerepresents a set of points in space which resembles a portion of a planein the neighborhood of each of its points, and is the case if thesurface is the image of a sufficiently regular mapping of a set ofpoints in the plane into E³. To which, and for the sake of simplicity, asurface is taken to be a point of reflection, be it on a closedconnected surface such as a sphere or torus, or on a surface patch. Forthe purpose of this invention, a reflective surface is therefore deemedto be a patch on which photons are reflected, be that patch part of aconnected surface such as the interior of a sphere or a cylinder, orpart of a disconnected surface separate from any other surface.

For the sake of simplicity, let all angles and all space coordinates berelative to the device and plane of the device within an xyz coordinatesystem in E³.

FIG. 1 is a block diagram depicting the basic device architecture 100.The device is comprised of at least one Photon Source 102, at least onePhoton Detector 103, where necessary, at least one Input FocusController 104, where necessary, at least one Output Focus Controller105, and at least one Plateau Region 101. It should be noted that notall geometries pertinent to this invention will require the use of 103and 104.

FIG. 2 is a block diagram depicting the basic Input Focus Controller104. The purpose of 104 is prevent newly injected photons fromre-entering 203, and/or interfering with the injection process. Withoutsuch a mechanism, the angle of reflection for photons entering theplateau region 101, would be too large to facilitate a suitable storagemechanism. Thus, 104 is comprised of a substrate 201, used to house thedevice, a mirrored surface 202, a curved mirrored surface 204 with adecreasing rate of curvature heading toward the Plateau Region 101, anda Photon Inlet Aperture 203. Photons 205 from a suitable Photon source102, are injected into 203, with an angle φ to the vertical plane of thedevice. φ is the Source Tuning variable.

From FIG. 2 a the law of reflection states that the angle of incidenceof a wave or stream of particles reflecting from a boundary,conventionally measured from the normal to the interface (not thesurface itself), is equal to the angle of reflection, measured from thesame interface.θ_(i)=θ_(r)   1

The law of reflection holds for both flat and curved reflectivesurfaces, where in a curved surface the tangent to the curved surface istaken as the flat boundary to the incident wave source.

As such from FIG. 3, the tangent 301 and normal 302 at the point ofreflection for the incoming photons 205 are effectively rotated throughan angle α, where α is related to the curvature of 204 at the point ofreflection. By inspection, it can be seen that the new angle ofincidence to 204 after reflection is:θ_(i)=φ−α  2

where φ is the initial angle of 205, and α is the effective angle ofrotation caused by the curvature of 204, and measured from the normal of204 against the normal of 205. The point on 204 chosen for the incident205 is such that α<θ.

As such the angle of reflection will now be, from equation 1:θ_(r)=φ−α  3

FIG. 4 shows the reflected photons 401 with the new angle of incidenceand angle of reflection as shown by equations 2 and 3. However, when thephotons strike 204 again, there will be a further decrease in θ_(i) fromthe new rotation value α′, again obtained from the tangential normal for204 against the normal for 401, a result of 204's changing curvature. Asbefore, the point on 204 for the incident 205 is such that α+α′<θ_(i).There is therefore a relationship between each subsequence angle ofincidence θ and curvature C of 204 at the point of incidence such that:θ∝C   4

where as C→0, θ→0 and as C→∝, θ→∝

Therefore and in general, photons move across the surface of 204, withthe curvature of 204 decreasing. As the curvature of 204 decreases, therelative angle of incidence for the next reflection decreases. This inturn will move the reflected photons to a new position along 204 toexperience yet a smaller still angle of curvature, so even furtherdecreasing the angle of incidence for the next reflection. Thiscontinued decrease in the relative angle of incidence continues until adesired angle of incidence is reached at the entry point to the Plateauregion 101. In so doing, and with each reflection, the relative stepsize, that is the distance between two successive reflections on thesame surface, will decrease in proportion to the decrease in the angleof incidence. This can be expressed first by relating the focus angle tothe sum of all incremental changes in α as a limiting case:$\theta_{f} = {\phi_{i} \pm {\sum\limits_{0}^{n}\alpha_{n}}}$

where φ_(i) the initial incident photon angle at 203, n is the number ofreflections for the incident photons to reach and enter 101, and α_(n)is the angle of effective rotation at each point of reflection, appliedto the incident photons as a direct result of the changing curvature of204. The ± is dependant on the focus control input (−) and the focuscontrol output (+).

The limiting factor is $\phi_{i} > {\sum\limits_{0}^{n}\alpha_{n}}$

such that θ_(f) is always positive and non-zero. θ_(f) is the criticalangle used in conjunction with other variables, to adjust the volume ofphoton storage within the device.

The iterative process of moving the photons along 104 can also beexpressed as:θ_(fn)=θ_(fn-1)±α_(n-1)

where θ_(fn) is the angle of reflection from 204 after the n^(th)iteration. Again the ± is dependant on the direction of curvaturerelative to photon movement, the focus control input is (−) and thefocus control output is (+).

As already discussed, the same principles and equations are applicableto photons entering the device in FIG. 2, using the focus inputcontroller, or the converse where photons exit the device using thefocus output controller in FIG. 6.

Adjustments to the focusing of 104 can be made by changing the initialangle of incidence φ for 205, changing the location of the firstreflection point on 204 or by changing the curvature of 204. The curveused in the above example for 204 is based on an ellipse.

FIG. 2 b shows a variation for 104 using two curved mirrors to focus205. This increases the rate of tuning 104. Now at each point ofreflection the angle of rotation induced by the curvature of 204 occurswith each reflection instead of every other reflection as with FIG. 2.

FIG. 2 c shows a further variation for 104 using a partial mirror as themeans to focus the injected photons. In this instance, the partialmirror allows photons to enter the device through the Photon InletAperture 203, but does not allow the reflected photons from 204 c tore-enter 203. It is suggested that such arrangement would still requirethe Output Focus Control as shown in FIG. 6.

In FIG. 5, the plateau region 101 is where the bulk of the injectedphotons are stored. The structure itself is comprised of two opposingreflective surfaces housed on a substrate 201, that store photons byreflecting an input photon first off one surface, then off an opposingsurface. This process is repeated until the photons effectively walkthemselves out of 101 with a step size governed by the angle ofincidence/reflection set on entry to 101 by the Input Focus Controller.

The surfaces of 101 need not necessarily be parallel, nor the angle ofincidence constant. However, photons moving from one surface to anotherwith each reflection should at no time touch the same xyz spacecoordinate twice, within the device and with each refresh of the device.

The amount of photons that can be stored by this device is thereforegoverned by three principle factors. The Plateau Length 501, the PlateauWidth 502 and the focused angle of incidence of the source photons 205on entering 101. The latter of these values is calculated from equation5.

Ordinarily, it is not feasible to calculate the number of actual photonsstored within the device. However, we can calculate data storagecapacity given the frequency of 205's oscillation, with the assumptionthat one bit of information is stored with each pulse of 205, the valueof the Plateau Regions height P_(h) and width P_(w). As such, we cancalculate the volume of data stored as:

Using FIG. 5 a, the distance s traversed lengthways in each half of areflective step by photons within 101 is: $\begin{matrix}{{\tan\quad\theta_{f}} = \frac{s}{P_{h}}} \\{s = {\tan\quad\theta_{f}\quad P_{h}}}\end{matrix}$

The distance traveled from one surface to another is, from simplePythagoras:l=√{square root over (P _(h) ² +s ² )}

Total distance traveled by photons within the plateau region istherefore the length of 101 divided by the step size distance (2 s)between each successive reflection on the same surface, multiplied bytwice the distance traveled (2 complete reflections) from one surface tothe other. Total distance is shown as: $d_{t} = {\frac{P_{w}}{2s}2l}$

On substitution and in terms of θ_(f) this gives:$d_{t} = {\frac{P_{w}}{\tan\quad\theta_{f}}\sqrt{( {1 + {\tan^{2}\quad\theta_{f}}} )}}$

and in terms of s gives:$d_{t} = {\frac{P_{w}}{s}\sqrt{P_{h}^{2} + s^{2}}}$

Given a data frequency f data storage volume can be calculated as afunction of the frequency or rate of data pumped into the system withina given time t.$V_{d} = {{ft} = {\frac{{fd}_{t}}{c} = {\frac{{fP}_{w}}{cs}\sqrt{P_{h}^{2} + s^{2}}}}}$

FIG. 6 shows the Output Focus Controller 105, essentially a reverseprocess to that employed for the Input Focus controller 104. A curvedreflective surface 204 with an increasing rate of curvature leading awayfrom the plateau region 101 slowly increases the angle ofincidence/reflection of photons that have propagated through 101. Asphotons exit 101, their reflected angle, gradually increases along withthe curvature of 204 until a suitably placed photon detector 103 at thePhoton Outlet Aperture receives the internally stored photons.

As storage duration within the device is limited by the dimension of101, re-circulation of the stored photons using a delay line principlemay be necessary to facilitate an effective store of data. Each newinput of photons to the device constitutes a device refresh.

The actual reflective surface used by this device can differ in a numberof ways. In FIG. 7, there are 3 principle surfaces that can be used.FIG. 7 a, a flat surface, FIG. 7 b a concave surface and FIG. 7 c aconvex surface.

This invention utilizes the storage of photons between one or aplurality of reflected surfaces. As such a number of possible curves andconfiguration types are suitable both in a single reflective plane oracross multiple reflective planes. Not all geometries will require aninput focus control 104 or an output focus control 105. A description ofvariation follows.

FIG. 8 a—Single continuous surface in the xy-plane and one of thereflective surfaces from FIG. 7 in the z-plane. Figure in 8 a show thatof a ring, but any continues surface in x, y will suffice.

FIG. 8 b—Double continuous surface in the xy-plane. This geometry showsa configuration that has one inner reflective surface and one outerreflective surface in the reflection plane. The figure in 8 b shows atorus, but any continuous outer surface in x, y and any continues innersurface in x, y will suffice. Both inner and outer surface would use oneof the surfaces in the z-plane, as shown in FIG. 7.

It should be noted that the geometries shown in FIGS. 8 a and 8 b do norequire that photons travel from one surface to another in the sameplane, and that to a limited degree imposed by the height of the surfacein the z-plane, photons may be able to travel out of the xy-plane andinto the z-plane between different surfaces.

FIG. 8 c—Single hollow surface in xyz. This geometry is in effect ahollow structure, such as a sphere, cone, cylinder or torus, with areflective surface on the inner wall.

Such geometries would allow photons to reflect beyond the single plane,so extending the geometric distance traveled while in the storagedevice. In the example shown in FIG. 8 c, only one reflection is shownin the cross section. All other reflections occur at different locationson the cross section for this example.

Multiple source and detectors: There is in general no limit to thenumber of photon sources and photon detectors for any given designgeometry, save for the actual physical space required for theirplacement.

Size and dimensions: There in general no limit to the size and geometryof this device save for the physical limits of current productiontechnology.

1. A photon storage device comprising: one or a plurality of reflectivesurfaces for the reflection of photons and so with each reflection theextension of the distance traveled, and hence a delay in time, for eachsaid photon within the said photon storage device; one or a plurality ofphoton sources for receiving an electrical signal representing data andinjecting said data, as photons, into the said photon storage device;one or a plurality of photon detectors for selectively detecting saidphotons traveling within said photon storage device; where necessary forthe said photon storage device geometry, one or a plurality of focuscontrol units of the curved surface type comprising: of one or aplurality of curved reflective surfaces where the curvature is of a typeso as to increase or decrease the relative angle of incidence and hencethe relative angle of reflection of photons within the said photonstorage device; and where necessary for the said photon storage devicegeometry, one or a plurality of focus control units of the partialmirrored surface type comprising: one or a plurality of partiallyreflective mirrored surfaces.
 2. The reflected surfaces within thephoton storage device of claim 1, can be singular and continuous, suchas the interior of a hollow sphere, or can be comprised in parts fromseparate discrete surfaces opposed to each other in the same geometricplane or in different geometric planes, such a reflective surface orcollection of reflective surfaces is called the main storage area of thesaid device.
 3. The main storage area of claim 2 is comprised of: two ormore opposing reflective surface boundaries such that photons strikingan (xyz) coordinate space of one surface boundary, will on reflectionproceed unheeded to intercept and make contact with a different (xyz)coordinate space of the said opposing surface.
 4. The surfaces of claim3 are highly reflective so as to reflect photons with limited photonloss and attenuation.
 5. That on entry to the main storage area of claim3, said photons are reflected repeatedly and unheeded to the same orother surfaces within the said storage area, until such time that thesaid photons exit the main storage area of claim 3, photons moving fromthe entry of the said storage area to the exit of the said storageconstitute a single pass.
 6. That photons traveling within the mainstorage area of claim 3, do not strike the same (xyz) coordinate spaceof the said device within a single pass.
 7. That for certain discretegeometries used within the photon storage device of claim 1, a focusinput controller is used at the said device input, after the photonsource 102, but prior to photons entering the main storage area of thedevice of claim
 3. 8. That the focus input controller of claim 7 iseither a reflective surface with a constant or variable curvaturecapable of decreasing in magnitude the said photon's angle ofincidence/reflection on entering the main storage area of the device ofclaim 2, or is a partially reflective mirror adjacent to the PhotonInlet Aperture, capable of allowing for a preset desired angle ofincidence/reflection on entering the main storage area of the device inclaim
 2. 9. The surfaces of claim 7, unless otherwise specified, arehighly reflective so as to reflect photons with limited photon loss andattenuation.
 10. That for certain discrete geometries used within thephoton storage device of claim 1, a focus output controller is used atthe said device output, after the main storage area of claim 3 but priorto the photon detector of claim
 1. 11. That the focus output controllerof claim 10 is a reflective surface with a constant or variablecurvature capable of increasing in magnitude the said photon's angle ofincidence and hence angle of reflection on leaving the main storage areaof the device of claim
 2. 12. The surfaces of claim 10, unless otherwisespecified, are highly reflective so as to reflect photons with limitedphoton loss and attenuation.
 13. The photon storage device of claim 1,wherein said photon source comprises a laser or light emitting diode.14. The photon storage device of claim 1, wherein said detectorcomprises a phototransistor, photodiode, photoresistor or photoreceptor.